This disclosure concerns the required backhaul capacity by the cooperative base stations for enhancement of uplink transmission.
In the present context, the backhaul portion of the network comprises the intermediate links between the core, or backbone, of the network and peripheral sub networks at the edge of the entire hierarchical network. For example, while cell phones communicating with a single cell tower constitute a local sub network, the connection between the cell tower and the rest of the world begins with a backhaul link to the core of the telephone company's network via a point of presence. Further, the expression uplink is here used to denote the transmission from a user equipment to a base station while a signal transmission in the opposite direction i.e. from the base station to the user equipment is referred to as a downlink transmission.
In design of cellular systems, or wireless communication systems as they also may be referred to, one of the challenging problems is to improve the performance of cell edge user equipment units. In cellular systems with frequency reuse, it is likely that user equipment units in the neighbouring cells transmit on the same resources as some of the user equipment units within the cell of interest. The simultaneous transmission by these user equipment units on the same resources creates interference which has a larger impact on cell-edge user equipments, since they originally suffer from larger attenuation on their transmitting signals to the base station. Hence, solutions to improve the performance of such user equipment units are of great interest.
Among various approaches to tackle this problem, some solutions promote cooperation between base stations by exchanging information between them to enhance the link quality which in particular would be beneficial for the cell edge user equipment units. The cooperation can be centralized or distributed. In the centralized approach a single super base station and remote antennas which are located in different cells are involved. The interference between cells with the same super base station can be mitigated, while the interference between cells belonging to a different super base station still remains.
An example of the distributed solution for cooperative base stations is based on the concept of letting the base station that is serving a certain user equipment receive complement information concerning the user equipment signals from one or more supporting base stations. Thus the serving base station could increase the likelihood of decode the user equipment signals correctly, thereby achieving less retransmissions. It may here be mentioned that the any base station of the cooperative base stations may sometimes have the role of serving base station and sometimes of supporting base station, depending e.g. of the position of the user equipment and/or load distribution.
The distributed solution tackles the drawback of the central solution while providing similar gain as the centralized solution at the expense of demanding higher capacity on the base station to base station interfaces whose traffic is carried on the backhaul network. Therefore methods to reduce the requirement on the backhaul capacity are of interest.
Further, wireless communication systems may employ error correction in order to correct errors caused by e.g. disturbance generated during transmission. A wireless communication system may use e.g. a turbo code for the error correction. At the transmitter side, the turbo encoder introduces redundancy bits based on the information data bits that are to be transmitted. The encoded data bits at the output of turbo encoder are then modulated and transmitted to the receiver, i.e. base station in uplink transmission. At the receiver end, the receiver demodulates the received signal and produce received soft bits to the turbo decoder. Turbo decoder decodes the received soft encoded bits to recover the information data bits. The concept of soft bits and hard bits are further explained later in the description.
To maximize the advantage of the coding gain obtained by the iterative decoding process in the turbo decoder, rather than determining immediately whether received encoded bits are zero or one, the communication receiver may assign each bit of value on a multi level scale representative of the probability that the bit is 1. This likelihood value is in the present context referred to as a soft bit, while the decoded data bit with a determined value, one or zero, is referred to as a hard bit. Thus the term hard bits is in this context meaning that the decoder decides a transmitted data bit is “0” if its corresponding measure indicates it is more likely that “0” is transmitted than “1” and vice versa. These bits are referred to as decoded hard data bits. Thus, to mention an example for binary signalling, received pulses are sampled and the resulting voltages are compared with a threshold value. If a voltage is greater than the threshold value it is considered to be definitely one, regardless of how close it is to the threshold value. If it is less than the threshold value, it is decoded definitely as zero.
A distributed control, based on the request-response mechanism has been proposed. A serving base station associated to the user equipment requests information from the supporting base station. In response, the information is transmitted to the serving base station via the backhaul network. Depending on the type of information, the required capacity on the backhaul network as well as the link quality for the user equipment in hand varies. The information obtained from the physical layer is in one of the following forms I/Q samples, Demodulated soft bits or Decoded hard data bits.
I/Q Samples
The In-phase and Quadrature-phase (I/Q) samples at the supporting base station associated to the user equipment of interest are transmitted to the serving base station in response to the request initiated from the serving base station. This exchange results in virtually increasing the number of receive antennas at the serving base station. The received signals from the serving base station and the supporting base stations can be processed jointly by means of advanced receiver algorithms for mitigating the interference and other impairments and provide improved link quality as compared to the non-cooperative case.
Demodulated Soft Bits
In this mode, in response to the request from the serving base station, the supporting base station processes the received signal with its corresponding receiver algorithms. The receivers can deploy methods for mitigating the interference as well as channel estimation. The processed received signals at different receive antennas are combined based on the utilized combining algorithms and are equalized. Eventually the combined signal is demodulated. The outcome is soft bits corresponding to transmitted coded bits which can be used as input values by the decoder. However, the supporting base station does not perform any decoding and instead transmits the demodulated soft bits to the serving base station before decoding. The serving base station which has performed similar operations on its own received signal combines the received demodulated soft bits from the supporting base station with its own, using for example chase combining method and then performs decoding.
Decoded Hard Data Bits
In this mode, in response to the request from the serving base station, the supporting base station processes the receive signal at its antennas even further than the previous case in the sense that the demodulated soft bits are decoded by the channel decoder and are also checked by the Cyclic Redundancy Check (CRC) decoder for error detection purposes. If the decoded hard data bits pass the CRC check, they are transmitted to the serving base in response to its request. The serving base station which has performed the similar procedure on its own received signals performs selection combining. In other words, the serving base station uses the decoded hard data bits received from the supporting base station only if its own decoded hard data bits are failed at the CRC check.
The last two mentioned modes can be also combined such that in response to the request from the serving base station, the supporting base station transmits the demodulated soft bits if the decoded hard bits are failed at the CRC check. Otherwise it sends the decoded hard data bits. In this case the advantages of the two modes are captured.
It may be mentioned that both I/Q samples and the demodulated soft bits are quantized and represented by bits before being exchanged. Apparently quantization is not required for the decoded hard data bits.
As it is clear, the amount of information in the I/Q samples is more complete as compared to the demodulated soft bits and the decoded hard data bits. The cooperation between base stations based on exchanging the I/Q samples virtually increases the number of receive antennas at the serving base station. This enables the serving base station to apply more advanced receiver algorithms to mitigate the interference and consequently improve the link quality.
When the demodulated soft bits are exchanged, the channel estimation, interference mitigation and equalization take place at each base station individually. The serving base station combines the received demodulated soft bits from the supporting base station with its own and performs decoding where some improvement is obtained due to the soft combining. The cooperation based on exchanging the demodulated soft bits in turn outperforms the cooperation based on the exchange of decoded hard data bits where the serving base station selects the decoded hard data bits sent by the supporting base station if its own have failed the CRC check.
However, the performance gain is at the expense of a higher demand on the backhaul capacity. The exchange of I/Q samples requires larger backhaul capacity as compare to exchanging the demodulated soft bits. The least capacity is required when decoded hard data bits are exchanged.
Although the best link quality performance can be achieved by exchanging the I/Q samples, the load on the backhaul capacity is considerably larger than the other options. Moreover, as the number of uplink user equipment units and/or the receive antennas and/or the supporting base stations grows the load on the backhaul capacity linearly increases. The severe impact on the backhaul network for the I/Q samples exchange makes the realization of this cooperation very inefficient in most practical cases. Therefore, from a practical point of view, the other two options that demonstrate lower backhaul capacity become more attractive, but at the cost of lower link quality. However, by increasing the number of user equipment units and/or supporting base stations the required capacity of these methods linearly increases as well.
Exchanging the demodulated soft bits especially when high order modulation schemes are used requires considerable backhaul capacity. Exchanging the decoded hard data bits provides the lowest load on the backhaul network but suffers from poor performance.